Ukraine has deployed troops near the Sea of Azov "in response to Russian aggression,” a top Ukrainian army commander has said following months of tit-for-tat arrests of merchant ships.

Although both countries enjoy free use of the Sea of Azov under a 2003 agreement, tensions heightened after Ukraine’s coast guard detained a Russian fishing boat in March and Russia opened a bridge to the annexed Crimea peninsula in May. Since then, Russia has reportedly seized or suspended at least 148 Ukrainian and foreign merchant ships while Ukraine has imposed sanctions on hundreds of Russian individuals and companies.

A week after Ukraine’s top national security body ordered to increase the country’s naval presence in the Sea of Azov, Ukrainian Ground Forces Commander Colonel General Serhiy Popko announced on Tuesday that reinforcements had been sent to the area.

“An armed forces group, including ground forces, has been enhanced in response to Russia’s aggressive actions in the Sea of Azov at the decision of the general staff,” Popko wrote on Facebook.

Territorial defense, missile and artillery forces, and army aviation have set up a permanent presence in the Sea of Azov “to provide reliable coastal defense,” Popko said.

Some Russian ships have stopped transporting cargo to Ukraine, deeming it risky after a Russian tanker was detained in a Ukrainian port as Kiev made good on sanctions against Moscow, two sources at shipowners told Reuters last month.

It is the latest fallout from Russia’s annexation of the Crimean peninsula in 2014 and the emergence of a pro-Moscow insurgency in eastern Ukraine.